The present invention relates to a cathode for an electron gun and, more particularly, to a quick operating type cathode for an electron gun of a cathode-ray tube.
In order to project an image on the screen of a television receiver as fast as possible and quickly stabilize the image projected on the screen, it is neccessary to adopt a quick operating type cathode as the cathode of an electron gun.
An example of a conventional quick operating type cathode will be described, together with the related structures, referring to FIG. 1.
A quick operating type cathode comprises a cathode body 5 consisting of a cathode sleeve 6 arranged in opposition to a first grid 1 supported by a first grid holding strap 2, and a metal substrate 7 coated with an electron-emissive material on a surface which opposes the first grid 1; a cathode body support cylinder 8 for supporting the cathode body 5 from, for example, three directions, through a support bar 9; a cathode holding strap 4 fixed to the cathode body support cylinder 8 through a cathode support 3; and a heater 13 supported by a heater strap 10 through a heater support plate 12. The straps 2, 4 and 10 are fixed at equal intervals by a pair of insulating support rods 11. The insulating support rods 11 are fixed to a stem 14 by support members.
A quick operating type cathode of this construction is assembled in an electron gun assembly, this electron gun assembly is sealed within a bulb, the bulb is evacuated, and aging is performed to complete a cathode-ray tube. The time required for an image to be projected on the screen from the time the switch of a sufficiently cooled television set provided with the cathode-ray tube is turned on is referred to as the image projecting time. This image projecting time is 4 to 4.5 seconds in a general rapid operating cathode-ray tube which assembles the cathode shown in FIG. 1 and which is sufficiently activated. However, such an image projecting time is not satisfactory. A cathode which further shortens the image projecting time will be described with reference to FIG. 2.
In the cathode shown in FIG. 2, the length of a cathode sleeve 16 is 1.5 times that of the cathode sleeve 6 of the cathode shown in FIG. 1. The cathode sleeve 16 is mounted to the cathode support cylinder 8 through a support bar 17. A coil part 18a of a double helical heater 18 is made shorter than the length of the cathode sleeve 16. With a cathode of this construction, since the heat generating part (coil part) 18a of the heater 18 is housed within the cathode sleeve 16, the heat efficiency is good. This results in an advantage in that the rate at which the temperature of the electron-emissive portion rises after the switch is turned on is faster than with the cathode shown in FIG. 1. The image projecting time may be shortened to about 2.7 seconds with this cathode.
However, with the cathode of the construction shown in FIG. 2, the outer diameter of a shoulder part 18c of the double helical coil necessarily becomes larger than that of the coil part 18a. For this reason, the shoulder part 18c and the inner wall of the cathode sleeve 16 tend to be abnormally close to each other, frequently degrading the voltage resistance between the heater and the cathode body. In general, with the heater 18, alumina is coated as an insulating material on the surface of a tungsten-rhenium alloy wire. This alumina generally contains about 0.1% of an alkali component, Na.sub.2 O. This alkali component is eliminated by evaporation by applying an excessive heater voltage to the heater during the aging process in the manufacture of the cathode-ray tube. However, with the cathode of the construction shown in FIG. 2, the alkali component evaporated from the heat generating part 18a at a high temperature is deposited on parts of the heater 18 at lower temperatures, i.e., leg parts 18d, the shoulder part 18c, and a final-turn part 18b. The deposition of the alkali component near the heater shoulder part 18c, and the abnormal closeness of the shoulder part 18c to the cathode sleeve 16, cause dielectric breakdown between the heater and the cathode body. When such a dielectric breakdown occurs, the heater potential interferes with the cathode potential, causing distortion of the image formed on the screen of the cathode-ray tube. When the dielectric breakdown is extreme, it may even cause disconnection of the heater.
A cathode as shown in FIG. 3 is conceivable as a quick operating type cathode which eliminates the defects of the cathode shown in FIG. 2. In the quick operating type cathode shown in FIG. 3, a heat generating part 19a is elongated so that the low-temperature parts of a heater 19, i.e., a final-turn part 19b, a shoulder part 19c, and leg parts 19d, are outside the cathode sleeve 16. For this reason, the defects of the cathode as shown in FIG. 2 are eliminated. However, with a cathode of such a construction, immediately after a voltage is applied to the heater, the temperature in the vicinity of the final-turn part 19b of the heater 19 is elevated first, and the temperature is then elevated at subsequent upper parts of the coil. This results in a disadvantage in that the elevation of the temperature of the cathode body, particularly in the metal substrate 7, is delayed. For example, the image projecting time of a television set which has assembled a cathode of the construction shown in FIG. 3 was found to be 3.2 seconds. There are other reasons for this delay in the image projecting time. For example, when the insulating material such as alumina is coated on the coil by the spray method, the insulating material is only thinly coated for 2 to 3 turns near the final-turn part 19b, but is coated thickly at parts closer to the turning point of the coil, i.e., closer to the metal substrate. To be more specific, the insulating material is sprayed onto the coil, with the edge portions, i.e., leg parts, of a numbers of coils arranged in series held by an instrument called "spray chuck." Thus, the spray stream is curved near the leg parts of the coil because of the presence of the spray chuck, leading to the above-noted difficulty.